﴿ وَأَخِى هَٰرُونُ هُوَ أَفْصَحُ مِنِّى لِسَانًۭا فَأَرْسِلْهُ مَعِىَ رِدْءًۭا يُصَدِّقُنِىٓ ۖ إِنِّىٓ أَخَافُ أَن يُكَذِّبُونِ ﴾

“And my brother Aaron - he is far better in speech than I am. Send him, therefore, as a helper, so that he might [more eloquently] bear witness to my speaking the truth: for I fear indeed that they will give me the lie.””

Please check ayah 28:35 for complete tafsir.